Delamination of a packaged IC die from its mounting surface, which is commonly triggered by die attach stress, is a known problem. One type of die attach stress (generally referred to as “differential coefficients of thermal expansion (CTE) stress”) is induced by heating (or cooling) a packaged IC die that arises from forces produced by the large differential CTE between the IC die and the die mounting surface (e.g., a die attach pad) that is often present, such as when the IC die comprises silicon and the die pad comprises copper.
A window pad type of lead frame is a type of lead frame that is used for construction of electronic packages. This type of lead frame is characterized by an opening (or “window”) located in the center of the die pad. Due to the reduced contact area between the IC die and the window pad lead frame, such arrangements help to prevent delamination of the mounted IC die from the lead frame. However, such arrangements generally have poor thermal performance due to the high thermal resistance of the epoxy-mold compound of the package that is between the IC and a heat sink (e.g. the ambient) in such arrangements.
To address thermal performance, some packages provide direct exposure of a portion of the IC die (e.g., the bottom of a silicon die) to an external heat sink (e.g. to a mother board or to the ambient), to bypass the thermal resistance of the package and thus improve the thermal performance of the packaged IC. Such packages are generally referred to as exposed silicon packages. Conventional exposed silicon packages include a solid (window-less) die pad, wherein the top surface of the IC die comprising active circuitry is attached by a die adhesive (e.g., epoxy) to a die pad that has an area substantially less then the area of the IC die. The bottom of the IC die is substantially coplanar with respect to the outer surface of the package and is thus exposed for coupling to an external heat sink.
In one alternate arrangement, an exposed silicon package includes a window pad lead frame. In this arrangement, an opening is formed in the central portion of the die-attach pad as a die-mounting ring for exposing one side of an IC die. The IC die has a conventional uniform shape (i.e. the cross sectional area of the IC die is constant in its thickness direction). The exposed surface of the IC die is recessed with the respect to the outer surface of the package which provides a cavity region that allows external cooling media to directly contact the exposed side of the IC die through the cavity. The exposed surface of the integrated circuit die is cooled, for example, with a cooling fluid, a heatsink, or a thermo-electric refrigeration unit in contact with the exposed side of the IC die.
Exposed silicon packages have several known problems. Problems for conventional exposed silicon packages include bleeding of the die attach adhesive (e.g., epoxy) onto the top surface of the IC die which can result in bonding and reliability problems, as well as die tilt. Die tilt can cause leakage of molding resin commonly referred to in the art as “mold flush”, as well as die cracking. In addition, both conventional exposed silicon packages and the alternate arrangement described above do not provide a mold lock to the IC substrate (e.g., silicon), so that ambient moisture can enter the packaged IC and reach the IC die, and as a result can cause reliability problems including delamination.